The present invention relates to an airfoil apparatus for gas turbine engines and a method for fabricating such an apparatus.
Gas turbine engines typically include a number of airfoil structures that interact with fluids that pass through the engine. Some of those airfoil structures comprise portions of non-rotating stator (or vane) structures. Stator structures are often made from forged components that are installed between a pair of shroud (or casing) rings through brazed connections. Brazing is a convenient and effective technique for joining airfoils to shroud rings to fabricate the stator structure. However, brazing can form a relatively low-strength joint that may not withstand relatively high stresses at or near the braze location. In essence, brazing can produce joints that are not as strong as the forged material of the stator structure. In order to compensate for the lower mechanical properties of brazed connections, stablugs have been added to stator structures. Stablugs are thickened portions of the stator structure that help keep braze materials away from the airfoil, which is thin and typically experiences relatively high stresses during engine operation. Stablugs can take a variety of cross-sectional shapes, including “racetrack” shapes (i.e., having linear side portions and rounded end portions that generally do not match that of the stator structure) as well as “airfoil” shapes that generally correspond to the aerodynamic contour of the airfoil. Regardless of the cross-sectional shape, the stablug must extend into the main gas flowpath of the engine adjacent to the airfoil. For example, the stablug may radially extend 0.127 cm (0.050 inch) proud into the gas flowpath for any given stator, which for low aspect ratio airfoils used in new engine designs can be over 9% of the span of the airfoil into the gas flowpath.
The relatively thick stablugs that extend into the gas flowpath of the engine have an undesirable impact on engine performance and efficiency (e.g., measured in terms of pressure loss), especially with stators for which it is desired to have a relatively small span. The stablugs create flow blockage at the endwalls of the stator structure in the gas flowpath. Moreover, the presence of a stablug precludes the inclusion of any stator features at that location, which is at the outer diameter or inner diameter of the airfoil where the stator is attached to the shroud rings. However, other known possibilities present cost, reliability and assembly problems. For instance, simply omitting the stablug can cause the braze joint to incur higher stresses during engine operation. Some stablugs can be partially recessed (though not entirely recessed), but recessing the stablug cannot be accomplished with high solidity stator assemblies (i.e., those with low aspect ratios and high vane counts) and does not completely eliminate the inefficiencies associated with stablug use.